I have been posting on forums for years now and it just seems like the majority of newbs don't stick around no matter what. Whether they are here just to do some spamming or just browsing for the day. It just seems like no matter what the majority of them don't stick around anywhere. The ones that do stick around usually get involved in other threads fairly quickly it seems and are easier to spot. Just how I see it.
I do think a lot more members fade away than stay. It just makes the forum seem active and friendly when they get hellos and welcomes. It's that whole first impression thing.